Data centar cooling using multiple chiller system with r134a

Roundtable on climate & ozone-friendly technologies in refrigeration & air-conditioning
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ANNUAL MEETING OF THE REGIONAL OZONE NETWORK
FOR EUROPE & CENTRAL ASIA
BELGRADE, SERBIA, 10-13 MAY 2011

Highly Efficient Data Centar Cooling Using Multiple Chiller System
with Oil-Free Variable-Speed Turbo Compressors
Dipl.-Ing. Slobodan Pejković

Filter Frigo d.o.o.
Serbia – 11050 Belgrade
E-mail=filfrigo@EUnet.rs

ABSTRACT

Oil-free twin-turbo centrifugal compressor uses electromagnetic bearings and turbine variable-speed for
significant increase of energy efficiency at partial load compared to conventional oil-lubricated centrifugal
or screw compressors.
Each of the three water chillers installed in Telecommunication Data Centar 'Belgrade' in April 2008 has
two turbo compressors and total cooling capacity of 1.97 MW. Such design solution with oil-free
compressors was applied for the first time in Serbia as well as in the territory of ex-Yugoslavia. It has
been estimated that in the 2008 cooling season this solution achieved more than 35% energy savings
comparing to the previous solution with conventional centrifugal compressors.
This paper presents the results of preliminary measurement of the working characteristics of chillers and
EER-energy efficiency ratio for real working conditions.

Keywords
Turbocor compressor; oil-free centrifugal compressor; energy efficiency

INTRODUCTION

In the Telecommunication Data Centar 'Belgrade' (Figure 1), back in 1973, water chillers ware installed
(Figure 2) with the conventional oil lubricated turbo compressor. The water chillers used R 12 as
refrigerant.
The engineers from Telecommunication Data Centar 'Belgrade' requested to retrofit the worn-out water
chillers with new, state-of-the-art chillers with an environmentally friendly refrigerant. Due to disturbances
that the old water chillers made in the electric network, especially when starting up the compressors
(starting current of over 500 A) it has been decided to install water chillers with oil-free turbo compressors
with starting current less than 5 A, and R134a as a refrigerant. The advantages of these water chillers
over the conventional ones are:
High energy efficiency, especially during reduced part-load, by use of the oil-free compressors
with the variable-speed turbine and flooded evaporator,
More efficient heat exchange in the evaporator and condenser (with no oil film causing additional
heat resistances),

10. May 2011. Belgrade Page 1

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Higher evaporation temperatures, and therefore higher cooling capacity at the same
temperatures of the water in the evaporator,
Silent work and reduced vibrations,
Low starting current (2 to 5 A ),
Light compressor weight (up to 140 kg),
Precise temperature regulation thanks to software controlling the compressor.

Figure 1: Telecommunication Data Centar 'Belgrade'

Figure 2: The old water chillers


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BASIC CHARACTERISTICS OF MODERN WATER CHILLERS WITH OIL-FREE
TURBO COMPRESSORS

Basic characteristics that set aside this family of water chillers with water cooled condenser (Figure 3)
from the other water chillers with conventional centrifugal or screw compressors are:

Figure 3: New water chillers with Turbocor compressors

High energy efficiency
In air-conditioning systems, the water chillers operate a smaller part of their time in the maximum load
regime. Therefore, it is very important to know the seasonal energy efficiency ratio, which has been
defined as following:
ESEER - (European Seasonal Energy Efficiency Ratio) is used to define the average annual cooling
efficiency of a water chiller system, and represents a ratio of total cooling energy exchanged in the
evaporator during the year – seasonal use of water chiller, expressed in Wh, in relation to total
absorbed energy used in the same period, expressed in Wh. ESEER is an average value of EERcooling
(Cooling Efficiency Level) during the year, at different external air temperatures, and therefore in
different operating regimes of the device.
This index presents energy efficiency of water chillers in more realistic terms, when compared to the
EER, because time period during the annual operating season, in which the chiller works under full load,
is shorter. Therefore, manufacturers try to develop the devices that would be more efficient during
reduced load conditions. Chillers, with the same cooling efficiency level EER, do not need to have the
same ESEER. A chiller with higher ESEER is more energy efficient.
ESEER is a weighed formula enabling to take into account the variation of EER with the load rate and the
variation of air or water inlet condenser temperature.


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ESEER = 0.03 • A + 0.33 • B + 0.41 • C + 0.23 • D

Where:
A = EER at 100% of load C = EER at 50% of load

B = EER at 75% of load D = EER at 25% of load

In water chillers with water cooled condenser, the ratios are determined for different loads at different
entering water temperatures. For the minimum load (25%), water temperature of 18ºC is adopted, and for
the maximum load (100%), the temperature of 30ºC.

From the quoted above, one can conclude that during the ESEER calculations, the assumption is that the
water chiller works only 3% of time under full load, and 41% of time with 50% load.

From Figure 4, one can see that the water chiller with oil-free Turbocor compressor is more efficient than
the water chillers with screw compressor, especially at loads less than 75%.

Figure 4: Energy efficiency ratio of water chiller at different loads

Sound level of water chillers
Environmental protection is becoming increasingly important in the air-conditioning systems, and
therefore also for water chillers. Sound level of oil-free centrifugal Turbocor compressors, as well as
vibrations which spread from the compressor to the foundations and the bearing construction of the
facility, are much less than the sound level and vibrations originating during the work of water chillers
using screw compressors of the same cooling capacity.

Continuous regulation of the compressor’s cooling capacity
In modern water chillers, water temperature is controlled at the outlet from the evaporator by changing
the speed of the impeller (turbine) and the opening degree of the inlet guide vane - IGV at centrifugal
compressor’s inlet side (Figure 5).


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Impeller

IGV

Figure 5: Cross-section of Turbocor compressor Figure 6: Electromagnetic compressor bearing

In this way the work of the water chiller shall be precisely adapted to the air-conditioning system
dynamics.

Figure 7: Schematic diagram of basic compressor elements

Compressor of small size and small weight is equipped with two impellers driven by electric-motor with
continuous frequency regulation. High-speed variable-frequency operation maintains outstanding part-
load efficiency, compactness and extraordinary soft-start capacity. Compressor’s rotor shaft and
impellers levitate during rotation and float on a magnetic cushion (Figure 6). The rotor shaft position
(Figure 7) is monitored and self-corrected by 10 sensors. Each shaft shift exceeding 0.5μ shall be read
and position correction shall be made accordingly. This technology, which has been transferred from the
space research program, has many advantages:

There is no oil for compressor lubrication, which enables more efficient exchange of heat in
the evaporator and condenser (there is no oil film creating additional heat resistances).
Extended compressor life due to lack of existence of wear surfaces; no metal-to-metal contact
of rotating components.
There are no vibrations in any of the work phases of the water chiller, including start.
The compressor drive motor is a synchronous permanent-magnet brushless DC motor, with
the power of 105 kW, with the size of a conventional fan induction engine, and power of 0.75
kW. It is cooled directly with refrigerant (R134a), works at high speed (20000 to 48000 RPM),
and there is no need for “Soft start system”, and the motor is completely protected.


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Small dimensions and weight (just 140 kg). The screw compressors with the same cooling
capacity are up to 5 times heavier and up to two times bigger in size.
Low starting currents up to 5A, during compressor’s start, instead of 500-600A for screw
compressors of the same cooling capacity.

Shell-and-tube flooded evaporator
The evaporator of the water chiller is the shell-and-tube in its appearance, and is of a flooded by its
function. It has been made of steel shell and a bundle of copper tubes, mechanically expanded in the
steel pipe-plates. The shell is thermally insulated. Thanks to the special drop liquid separator system
there is no possibility for the compressor to suck in liquid refrigerant, and overheating of the refrigerant
at the inlet of the compressor is less than 3K. The level of the liquid refrigerant is controlled and
maintained with the help of special electronic level sensors and by a modular expansion valve. The
water circulates through the bundle of copper tubes, and the refrigerant (R134a) is around the tubes
(Figure 8).

Turbo
Compressor

Evaporator

Condenser

Figure 8: Principal operating scheme for water chiller

This is possible to execute, because there is no oil in the refrigerant, as in conventional oil-lubricated
compressors; oil runs away from the compressor and appears in the refrigerant side of the heat
exchangers (evaporator and condenser). In this way, the refrigerant boiling has been enabled throughout
the total external surface of the copper tube bundle. For this reason, the heat transfer ratio has been
significantly increased, and small temperature difference has been secured between the outlet water
temperature from the evaporator and the evaporating temperature (less than 2K). This results in
increased cooling capacity, with slight increase of absorbed power, leading to an increased energy
efficiency of the water chiller.


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Interactive remote control
By the installed software through local area network, or Internet, one can monitor the work of the water
chiller from any computer anywhere in the world (Figure 9).

Figure 9: Connecting compressor to a computer

THE MAIN PARTS OF OIL-FREE TWIN-TURBO CENTRIFUGAL COMPRESSOR

Figure 10: Turbocor compressor’s main parts


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FACTORY MEASUREMENT OF WATER CHILLER’S WORKING PARAMETERS

Expert team consisting of mechanical and electric engineers from Filter Frigo has attended factory
tests of the water chiller. The tests have been performed on April 3rd, 2008 in the test cabin no. 3
(Figure 11).

Table 1 shows calculated cooling capacity and absorbed powers at different loads of the water chiller
and different water temperatures for condenser cooling.

The results of the measurements for one of three water chiller have been presented in Table 2.

Figure 11: Test cabin no. 3

Water Chiller TECS-HF 2AS
Load % 100 66 50 33
Cooling capacity kW 657.4 433.88 328.7 216.94
Total absorbed power kW 128.4 57.5 34.2 20.7
EER Wh/Wh 5.11 7.55 9.61 10.47
Water entering the evaporator °C 14 12.3 11.5 10.6
Water exiting the evaporator °C 9 9 9 9
Water flow through the 3
m /h 113.2
evaporator
Water entering the condenser °C 29.5 26 22 18
Water exiting the condenser °C 34 28.8 24.1 19.4
Water flow through the
m3/h 149.7
condenser
Table 1: Calculated operating parameters of the water chiller


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Table 2: Original factory testing report of the water chiller TECS-HF 2AS

FACILITY REQUIREMENT FOR COOLING

For cooling of equipment and air-conditioning in the building of Telecommunication Data Centar
'Belgrade', a total cooling capacity is needed of 1.2 MW. For those requirements, three water chillers,
with water cooled condensers were selected, each with cooling capacity of 658 kW, in the following work
regimes:
Water temperature at the evaporator inlet 14ºC
Water temperature at the evaporator outlet 9ºC

Water temperature at the condenser inlet 29ºC
Water temperature at the condenser outlet 34ºC
The installed capacity is much higher (1.97 MW) than needed, due to the reason of redundancy
safeguards. Processor (Manager 3000, shown in Figure 12), controls the work of all three water chillers.
The simultaneous work of all three water chillers was enabled at reduced load, when the electric power
consumption has been significantly decreased.
Regulation of the cooling capacity of the water chillers (from 10% to 100%) is executed in two ways:
By changing the speed of the turbine, according to factory data from 20000 to 48000 min¯¹.
Measured from 22500 to 34000 min¯¹.
By changing the opening degree of the inlet guide vane - IGV at the inlet side of the compressor.

It was found that combined regulation has a good performance.


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Figure 12: Processor for controlling the work of three water chillers

When two water chillers operate at high external temperatures, in order to satisfy the cooling requirement
(Figure 13), their total cooling capacity is 2 x 612.6 = 1225.2 kW, while the absorbed power is 2 x 139.8
= 279.6 kW. The cooling capacity is less than nominal, and the absorbed power is higher, due to higher
temperature of water for cooling of the condenser. The condensing temperature is tk = 40ºC (Table 4,
page 16, the first column).

When all three water chillers operate at decreased speed in order to satisfy the same cooling
requirement, their total cooling capacity is 3 x 407.5 = 1222.5 kW, while the absorbed power is 3 x 64.0 =
192.0 kW, that is 31% less. The condensing temperature is tk = 32ºC (Table 3, page 15).

The use of Multiple Chiller System allows unusual redundancy safeguards.

100% 100%
33.3% 31%
80% 50% 50% 80%
23%
60% 33.3%
60%
40% 23%
40%
50% 50% 20% 33.3%
23%
20%
0%
0% Cooling capacity Absorbed power
Cooling capacity Absorbed power
Water Chiller I Water Chiller II
Water Chiller I Water Chiller II Water Chiller III Energy savings

Figure 13: Comparative presentation of energy consumption, if two or three water chillers
are operational for the same cooling capacity


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These values have encouraged us to determine the annual seasonal energy consumption by calculation
for the data center cooling, using multiple chiller system.

Three cases have been discussed in order to be able to determine an optimum operating method for the
system. Cooling of equipment in the winter is working in the free cooling mode.

The first case is the analysis of the work of an old Westinghouse water chiller, with cooling capacity of 1.2
MW, with conventional oil-lubricated centrifugal compressor. According to the method recommended by
KGH (Serbian Society for HVAC&R), we have assumed that the water chiller operates only 240 days a
year, 14 hours per day. The calculated seasonal electric power consumption is 571200 kWh during the
year. For securing the facility requirement for the cooling, only one water chiller was used, and the other
served as a reserve.
The second discussed case is a parallel work of two new water chillers with Turbocor compressors.
During that, the third water chiller serves as a reserve. For the same number of hours of work, as in the
previous case, the seasonal energy consumption is 364800 kWh, which is 36% less than in the first case.

The third case is the parallel operation of all three new water chillers. For the same number of hours,
seasonal energy consumption was 212600 kWh, which is 63% less than in the first case.

According to the world price lists for electricity utilization, the financial effects, that is, annual savings in
this operating regime of the water chillers would be $37650. The current project indicates a simple
payback for the third water chiller of approximately 2.5 years.

MEASUREMENT OF CHILLER WORKING PARAMETERS IN THE FACILITY
The measurements have been performed in August and on October 14th, 2008.

Measuring equipment
For basic measurement of temperatures and pressures, the measuring equipment which has been
installed in the factory in water chillers, has been used.
For control measurements of temperatures and pressures, the measuring equipment, product of
ROBINAIR, has been used.

For indirect measurements of the speed of turbine, opening degree of inlet guide vane - IGV and for
determining the requested load of the water chiller, a computer has been used (Figure14) with the
software package from »Danfoss Turbocor Monitoring Tool Version 2.1.0.80«.

For electrical network characteristics measurement, instrument CHAUVIN ARNOUX C.A 8334 has been
used (Figure 15).

For basic measurement of the water flow rate orifice meter have been used in conection with
OVENTROP flow-meter OV-DMC 2 (Figure 16).

For control measurement of water flow rate, the ultrasonic flow-meter UFM 610P has been used, product
of KROHNE (Figure 16).


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Figure 14: Measuring equipment Figure 15: Instrument
CHAUVIN ARNOUX C.A 8334

Figure 16: Instrument OVENTROP OV-DMC 2 and ultrasonic flow-meter KROHNE UFM 610P

By detailed measurement and analysis of current characteristics of the water chiller, the influence of the
variable speed electric motor has been observed to so-called “defilement“ of power grid.

From the voltage diagram (Figure 17), it can be observed that there is small voltage distortion because
voltage oscillograms are not ideal sinusoids.

From the current intensity diagram, one can observe that the currents in phases are symmetrical.
Simultaneously, this diagram shows that there is serious current distortion, because the current
oscillograms are not ideal sinusoids, but have in each semi-period two symmetrical peaks, resulting from
higher odd harmonics (Figure 17).

In other words, good characteristics, like soft start (maxigraph compensation) and speed regulation,
which is the compressor cooling capacity regulation, have for their consequence, the current and voltage
distortion. This results in an appearance of interferences in the form of higher harmonics of voltage and
current. In installations where such level of higher harmonics might cause problems, we use filters, based
on inductive chokes mounted in the compressor’s power line.


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Figure 17: Results of the electrical parameters measuring (power, cosφ, voltage, current intensity)

The measurements show that by decreasing the speed of the turbine, power factor - cosφ also
decreases, and it increases the percentage of reactive power in apparent power. Considering the costs of
the reactive power, reactive power should be compensated. On the other hand, low starting current (up to
5A) does not cause problems with »peaks« and it does not switch on maxigraph, which in conventional
water chillers with starting currents from 500A to 600A significantly influences the price of the consumed
electricity.

The software package »Danfoss Turbocor Monitoring Tool Version 2.1.0.80« offers possibilities for
monitoring different parameters in the work of Turbocor compressor. From the program the operator can
open up different windows, some of which were presented in the Figures 18, 19 and 20.

Figure 18


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Figure 19

Figure 20


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Table 3 shows the results of measuring the working parameters of water chiller no.2 at high external
temperatures (measured in August 2008). The water chillers no.1 and no.3 were in simultaneous
operation.

Water Chiller: TECS-HF 2S Compressors: Date:
Φo=658 kW ; Pw=130 kW ; tw=14/9°C TT300 (R134a) 12/08/2008.
Outside Air Temperature 37.9ºC

Parameters

Discharge Pressure [kPag] 702

Condensing Temperature [°C] 32

Discharge Temperature [°C] 38.5
3
Water Flow (Condenser) [m /h] 126.3

Water Temp. (Cond. Inlet) [°C] 25.7

Water Temp. Difference-Cond. [K] 3.1

Heating Capacity [kW] 455.3

Suction Pressure [kPag] 290

Suction Temperature [°C] 7.6
3
Water Flow (Evaporator) [m /h] 129.8

Water Temp. (Evap. Outlet) [°C] 9.0

Water Temp. Difference-Evap. [K] 2.7

Cooling Capacity [kW] 407.5
-1
Shaft Speed [min ] 27371

Compressor load [%] 34

IGV (Inlet Guide Vane) open [%] 110

Power Input [kW] 64.0
Cosφ - 0.893

EER - 6.4

Table 3: Results of measuring the working parameters of water chiller no.2

The compressors worked with completely opened inlet guide vane - IGV, and with reduced impeller’s
-1
speed of 27371 min . Because the condenser’s load was below 70%, this enabled the compressor to
operate at condensing temperature of 32ºC and the efficiency level - EER should be significantly
increased (6.4).

Table 4 shows the results of measuring the working parameters of water chiller no.2 at different loads.

During the measurement opening degree of inlet guide vane – IGV was changing from 110 to 8.8%, as
well as the compressor impeller’s speed from 33978 to 22520 min-1. The efficiency level - EER changed
from 4.4 at maximum load to 9.1 at reduced capacity of 34% of the maximum load. By further decrease
of load, the efficiency level - EER would also increase.


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Water Chiller: TECS-HF 2S Compressors: Date: Outside Temp.
Φo=658 kW ; Pw=130 kW ; tw=14/9°C TT300 (R134a) 14/10/2008. 21°C
Parameters / Time 12:48 13:08 13:16 14:15 14:34 15:32 15:37

Discharge Pressure [kPag] 887 793 769 702 618 613 595

Condensing Temperature [°C] 40 36 35 32 28 27 26

Discharge Temperature [°C] 50.5 47.4 41.2 36.2 33.8 32.8 32.0
3
Water Flow (Condenser) [m /h] 126.1 125.8 125.6 125.6 125.9 125.8 126.3

Water Temp. (Cond. Inlet) [°C] 26.6 25.2 25.4 24.3 24.0 23.5 23.1

Water Temp. Difference-Cond. [K] 5.0 4.7 3.7 2.6 1.8 1.5 1.4

Heating Capacity [kW] 733.1 687.5 540.4 379.7 263.5 219.4 205.6

Suction Pressure [kPag] 3.0 2.67 2.8 2.89 2.92 2.95 2.98

Suction Temperature [°C] 9.2 5.9 7.0 7.6 8.2 9.1 9.2
3
Water Flow (Evaporator) [m /h] 128.5 129.8 129.5 129.8 129.1 128.8 129.4

Water Temp. (Evap. Outlet) [°C] 11.3 8.7 8.7 8.9 8.8 8.8 8.9

Water Temp. Difference-Evap. [K] 4.1 3.9 3.2 2.2 1.6 1.4 1.3

Cooling Capacity [kW] 612.6 588.6 481.9 332.0 240.2 210.0 195.6
-1
Shaft Speed [min ] 33802 33978 29182 25434 23784 22907 22520

Compressor load [%] 90 74.8 42.8 26.8 10 10 11.7

IGV (Inlet Guide Vane) open [%] 110 110 110 110 23.6 8.8 8.8

Power Input [kW] 139.8 130.8 78.5 46.5 30.8 23.2 21.6
Reactive power [kVAR] 60.6 56.9 36.6 26.6 21.3 19.2 18.2

Cosφ - 0.917 0.916 0.906 0.867 0.822 0.771 0.762

EER - 4.4 4.5 6.1 7.1 7.8 9.0 9.1
Table 4: Results of measuring the working parameters of water chiller no.2

Pumps for circulation of cold (evaporator) and cooling (condenser) water

Each of the water chillers has its own pair of pumps. Pump for circulating water through the evaporator
shall start its operation before the start of the first compressor in the water chiller, and the pump for
circulation of the water through the condenser, shall start when the condensing pressure reaches the set
value. These pumps are important consumers of electricity and haven’t been calculated in when
determining the efficiency level - EER.


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CONCLUSION:

Oil-free twin-turbo centrifugal compressor uses electromagnetic bearings and a variable-
speed turbine for significant increase of energy efficiency at partial load compared to the
previous solution with conventional centrifugal compressor.

Water chillers with water cooled condenser offer high energy efficiency level - EER at the
highest external temperatures, because the water chillers operate with condensing
temperature up to 40ºC.

Part load energy efficiency is outstanding, with energy savings greater than 35% compared
to the conventional oil-lubricated centrifugal or screw compressors.

Totally oil-free operation has improved the heat transfer efficiency.

Extended equipment life was provided with minimal scheduled maintenance.

The current project indicates a simple payback for the third water chiller of approximately 2.5
years.

The Multiple Chiller System allows unusual redundancy safeguards with energy savings
greater than 35%.

Reduced energy consumption provides a way to reduce utility generated greenhouse gas
emissions.

Literature:

1. Određivanje, klasifikovanje i označavanje energetske efikasnosti agregata za hlađenje vode i
toplotnih pumpi,
Slobodan Pejković - Vladimir Živanović, Filter Frigo d.o.o., Belgrade

2. Danfoss Turbocor, USCO 7.2008

3. Water cooled liquid chiller SerieTECS-HF / TECS-HFD,
Cod: B100AS_120_220B_CV_06_06_IT_GB,
CLIMAVENETA

4. www.turbocor.com


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